1. Field of the Invention
This invention relates generally to digital subscriber lines (DSL""s), and more particularly to a low distortion, power efficient DSL/Cable line driver that employs DC biasing and impedance synthesis without using a series resistor to achieve impedance matching.
2. Description of the Prior Art
Asymmetric digital subscriber lines (ADSL) can supply the necessary bandwidth for applications such as fast access to the Internet, video conferencing, interactive multimedia, and Video-on-Demand. This technology is designed to solve the most severe bottleneck in the data access network between the Central Office and the customer, or end-user.
ADSL is a new modem technology that converts existing copper telephone lines into access paths for multimedia and high speed data communications, and maintains the regular telephone voice services. ADSL provides data rates that expand the best existing access capacity (i.e., ISDN) by a factor of 60 or more, and the best existing common analog access capacity (i.e., V.34 modems) by a factor of 300, without requiring any new cabling. ADSL can practically transform the existing public information network from one limited to voice, text and low resolution graphics to a powerful, ubiquitous system capable of carrying multimedia, including full motion video, to everyone""s home.
Rate adaptive ADSL delivers any data rate from 64kbps to 8.192Mbps on the downstream channels to the subscribers and any data rates from 16kbps to 768kbps on the upstream channels back to the network, while simultaneously providing lifeline POTS (Plain Old Telephone Service), all over a single twisted copper wire pair. The downstream and upstream channels can be split to several sub-channels (up to 4 sub-channels on the downstream and 3 bi-directional sub-channels) to serve several applications simultaneously.
The ADSL operates over a single, unconditioned twisted copper pair of wires. Its connection is via modem pairs, one at the user end and the second at the Central Office. The ANSI T1 committee has standardized Discrete Multi-Tone (DMT) as the line code to be used in the ADSL transmission system, due to its unique ability to overcome the severe distortion of the copper line at this frequency band. DMT also matches well the typical noise and impulses which exist in the residential twisted-wire pair environment. DMT divides the channel into a number of sub-channels, referred to as tones, each of which is QAM-modulated on a separate carrier. The carrier frequencies are multiples of one basic frequency. The available spectrum ranges from about 20 kHz to 1.104 MHz, while the low 20kHz are reserved for voice service (POTS). Noise and channel conditions are constantly measured for each tone separately to achieve optimal transmission at any time.
A common strategy for implementing ADSL modem technology is to ensure that the input impedance of an analog front end (AFE), looking into the subscriber loop port, matches the loop characteristic impedance. This has traditionally been accomplished by placing a network that closely matches the loop characteristic impedance between the AFE line driver output and the subscriber loop (or transformer coupling to the subscriber loop). A disadvantage associated with such schemes is that half of the line driver output signal power is dissipated on this terminating network. In view of the foregoing, a need exists for a low noise, power efficient, actively terminated line driver that avoids terminating network signal power loss. One method of active termination is disclosed in U.S. Pat. No. 4,798,982, issued Jan. 17, 1989 to Voorman, that is entitled Active Symmetrical Balance Hybrid Circuit. The scheme disclosed and claimed by Voorman, however, exhibits distortion that is too high for use in most DSL technologies. It is therefore desirable to provide a DSL line driver scheme that employs active elements, but which also reduces the size and number of passive elements to achieve reductions in both distortion and power dissipation.
The present invention is directed to a low distortion, power efficient DSL/cable line driver that is devoid of series resistors to accommodate impedance matching. The line driver further acts as part of line termination circuitry to achieve good side tone rejection and achieves very low to no DC loss in transformer connected architectures because of accurate feedback based DC level biasing. One embodiment of the invention combines active termination and sidetone cancellation in a double Wheatstone bridge architecture such that a transmit signal is cancelled at the input of the receiver amplifier and such that a received signal is terminated by the transmitter amplifier. The line driver further has a receiver amplifier that preferably employs a programmable resistor configured within a resistor network to optimize side tone rejection characteristics over a wide range of line impedances. The line driver employs both negative feedback and positive feedback in combination with DC biasing to minimize DC offset voltages occurring across an output transformer when the line driver is used in a differential application such as ADSL.
In one aspect of the invention, a DSL/cable line driver is implemented using a differential receive amplifier in combination with an actively terminated transmitter driver to effectively reject a transmitted signal while applying at least unity gain to the received signal.
In another aspect of the invention, a DSL/cable line driver is implemented using a differential receive amplifier in combination with an actively terminated transmitter driver to provide a line driver that is more power efficient than a conventional passively terminated line driver.
In still another aspect of the invention, a DSL/cable line driver is implemented using a differential receive amplifier in combination with an actively terminated transmitter driver using accurate feedback based DC level biasing to achieve very low to no DC loss in transformer connected architectures and good sidetone rejection.
In still another aspect of the invention, a DSL/cable line driver is implemented by combining active termination and sidetone rejection circuitry in a double Wheatstone bridge architecture.